1. Field of the Invention
The present invention relates to a molded abrasive product to be used in a mechanical polishing process or a chemical mechanical polishing (hereinafter referred to as CMP) process for polishing e.g. optical materials and substrate materials such as silicon wafers, oxide substrates, semiconductor substrates, glass substrates and ceramic substrates, and a polishing wheel using it.
2. Discussion of Background
Along with progress in e.g. optics and electronics industries, requirements for processing of e.g. magnetic disks, semiconductor substrates, single crystal materials and optical materials, have become very strict, and in the finishing of electronic parts, it has been required to make the surface of the material flat and smooth by polishing. Accordingly, a polishing treatment has heretofore been employed wherein the surface of the material is polished with a polishing pad (polishing cloth) made of e.g. nonwoven fabric or suede cloth, while a polishing liquid containing loose abrasive grains is continuously supplied thereon. As the loose abrasive grains, e.g. aluminum oxide, silicon oxide, cerium oxide, zirconium oxide, iron oxide, titanium oxide, manganese oxide or silicon carbide has been used.
However, by the polishing treatment using such loose abrasive grains, e.g. an edge of the material subjected to polishing (hereinafter referred to as material to be polished) may be polished too much during polishing since the surface of the polishing cloth used is soft, whereby the entire surface of the material to be polished can not be uniformly polished.
Further, with respect to the conventional polishing method using a polishing cloth, a polishing liquid containing no abrasive grains, such as water having its pH adjusted, can not be used in most cases, and accordingly, it is necessary to use a large amount of a polishing liquid containing loose abrasive grains. As a result, a waste polishing liquid containing a large amount of loose abrasive grains is produced, and its treatment has to be improved in view of efficiency of the polishing treatment, equi.mu.ments for the waste disposal and the influence over environment.
To overcome the above problem, e.g. JP-A-4-256581 proposes a polishing method of a fixed system using a composite grinding wheel having abrasive grains bound by a synthetic resin, for polishing. This method using the composite grinding wheel, is disclosed to be effective to overcome such a problem that the entire surface of the material to be polished can not be uniformly polished, which has occurred in the conventional polishing method using a polishing cloth.
However, with such a grinding wheel, the synthetic resin used as a binder is present also at its surface which takes part in polishing, whereby the same condition as so-called clogging tends to occur, thus decreasing the polishing efficiency and productivity. Further, since a synthetic resin is used as a binder, the material to be polished is likely to be contaminated by impurities from the composite grinding wheel during the polishing process, depending upon the polishing conditions.
Under the circumstances, the present inventors have found that, as disclosed in JP-A-10-264015 for example, a molded abrasive product containing, as the main component, silica which is abrasive grains, is applicable to the polishing process, and conducted studies to overcome the above object, and as a result, they have found the following.
1) Since the molded abrasive product has a large elastic coefficient as compared with a polishing cloth, a corner of e.g. the edge of the material to be polished will be rarely polished too much, and accordingly, the entire surface of the material to be polished will be uniformly polished.
2) The surface of the molded abrasive product, taking part in polishing, is rough due to the silica powder as the material, and many pores are present among the silica particles, whereby so-called clogging in the polishing process will be suppressed.
3) The molded abrasive product contains no resin component, whereby it has e.g. heat resistance, chemical resistance and water resistance during the polishing process, and accordingly, the polishing liquid may be used even at a high temperature in the vicinity of its boiling point, and the type of the polishing liquid may be optionally selected, so that the polishing process can be carried out under an optimum condition, thus increasing the polishing efficiency.
4) The molded abrasive product is made of silica which is used as abrasive grains, and the influence of impurities resulting from the molded abrasive product on the material to be polished during the polishing process can be suppressed.
5) The level of the finishing of polishing of the material to be polished can be polished is about the same as in the conventional method by using a polishing cloth, and the polishing rate is equal to or higher than that of the conventional method, and further, deterioration of polishing performance with time is small in the polishing process.
6) The surface of the molded abrasive product, taking part in polishing, is rough due to a silica powder as the material, and since said surface is directly in contact with the material to be polished, the molded abrasive product may be applied to a process for polishing e.g. substrate materials by using a polishing liquid containing no loose abrasive grains.
7) Even in a case where an abrasive containing loose abrasive grains, such as aluminum oxide, silicon oxide, cerium oxide, zirconium oxide, manganese oxide, titanium oxide, magnesium oxide, iron oxide, chrome oxide or silicon carbide, or a mixture thereof, is used, an adequately high polishing rate can be obtained with a low concentration of loose abrasive grains as compared with the conventional method of using a polishing cloth.
As mentioned above, the molded abrasive product as disclosed in JP-A-10-264015 by the present inventors, is very suitable for a CMP process and a mechanical polishing process for polishing e.g. optical materials and substrate materials such as silicon wafers, oxide substrates, semiconductor substrates, glass substrates and ceramic substrates. However, the higher the hardness of the material to be polished, the smaller the polishing rate tends to be, and accordingly, a molded abrasive product which increases the polishing rate of a particularly hard material having a Vickers hardness of at least 650 kg/mm.sup.2, has been desired.
Further, JP-A-10-337669 discloses a grinding wheel obtained by firing inorganic abrasive grains. It describes materials constituting the grinding wheel, the particle size, the porosity and the coefficient of water absorption, and describes that an effect similar to that in the above JP-A-10-264015 can be obtained. However, the surface precision of the silicon wafer as an example of the material to be polished, is such that the centerline average roughness is a level of 3 nm, and further, no mention is made of the polishing rate.
On the contrary, the above JP-A-10-264015 presents a result of measuring the surface precision of a silicon wafer by means of a universal surface-shape measuring apparatus. Since the apparatus has reached the limit in measurement, the present inventors have carried out also a measurement by means of an atomic force microscope (AFM), for more accurate measurement. As a result, it was clarified that the centerline average roughness is from 0.6 to 1 nm, and an excellent surface precision may be obtained.
However, such examples are mainly for polishing silicon wafers, and a molded abrasive product which has a performance to polish the surface of a material to be polished at a high rate with a high accuracy, and which will be useful for hard materials, particularly for hard materials having a Vickers hardness of at least 650 kg/mm.sup.2, has been desired. A molded abrasive product for polishing such a hard material, to industrially make the surface extremely smooth and flat, will be useful for polishing substrates for electric materials and substrates for optical materials.